This invention relates to a pneumatic radial tire having highly durable bead portions.
Heavy duty pneumatic radial tires to be used for trucks, buses and the like are particularly required to have high durability of bead portions. Such a requirement results from not only the severe service conditions but also the fact that they are generally used repeatedly by recapping every time tread rubbers have been torn off to realize their longest possible effective life.
With such heavy duty pneumatic radial tires, failures at bead portions are often found when they are inspected whether they are able to be used again by recapping. Such failures at the bead portions are mainly caused by high tensile strains in the proximities of turn-up ends of carcass plies, particularly radially outwards thereof. In most cases, the tensile strains give rise to rubber fatigue and separations.
In order to improve the durability of the bead portions, it has been proposed to provide reinforcing members or other particular members to mitigate the above strains and to restrain any growth of cracks due to the rubber fatigue. However, such an addition of the members detrimentally affects productivity of tires accompanying with increased cost and the occurrence of heat.
In general, the heavy duty pneumatic radial tire to be used for trucks and buses comprises a pair of bead portions, sidewalls and a tread extending between the sidewalls and further comprises as a body reinforcement a carcass extending between the bead portions and having plies whose cords are radially arranged. At least one ply of the carcass is folded from inside to outside about bead cores into a turn-up construction. In this case, the durability of the bead portions depends upon the turn-up ends of the carcass. However, direct measures for the turn-up portions to solve these problems have been insufficient to obtain useful effects.